The broad objective of this project are to study the stimuli that induce tolerance in normal T cells and in vivo, with emphasis on the roles of costimulators, antigen-presenting cells and cytokines, and to examine the tolerance sensitivity of different T cell populations, including cells that escape negative selection in the thymus. A major emphasis is on the use of T cell receptor (TCR) transgenic mice. The specific aims of the project are the following. 1. Tolerance in naive CD4+ T cells in vitro The development of tolerance will be studied in normal CD4+ T cells isolated from transgenic mice expressing a TCR specific for cytochrome c + I-Ek. The types of APCs that induce tolerance, and their costimulators, will be examined. The ability of cytokines to prevent tolerance will be studied. The functional impairment and fate of tolerant cells will be examined. 2. Mechanisms of tolerance induction in vivo Peripheral T cell tolerance will be induced in normal and TCR transgenic mice by high-dose aqueous antigen, antigen targeted to B cells, and oral antigen administration. The influence of cytokines on T cell tolerance will be examined with specific antagonists. The role of B cells as tolerogenic APCs will be studied in B-deficient mice. Thus, the is project addresses fundamental questions about the induction and maintenance of peripheral T cell tolerance in normal T cells and in vivo, and the physiologic significance of peripheral tolerance.